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Nanocrystal
A nanocrystal is a material particle having at least one dimension smaller than 100 nanometres (a nanoparticle) and composed of atoms (WP) in either a single- or poly-crystalline arrangement. The size of nanocrystals distinguishes them from larger crystals (WP). For example, silicon nanocrystals can provide efficient light emission while bulk silicon does not and may be used for memory components. When embedded in solids nanocrystals may exhibit much more complex melting behaviour than conventional solids and may form the basis of a special class of solids. They can behave as single-domain systems (a volume within the system having the same atomic or molecular arrangement throughout) that can help explain the behaviour of macroscopic samples of a similar material without the complicating presence of grain boundaries and other defects. Semiconductor (WP) nanocrystals having dimensions smaller than 10nm are also described as quantum dots. Synthesis The traditional way to prepare nanocrystals of a new material involved choosing molecular precursors, surfactants, and solvents using optimized reaction conditions causing the atoms to self-assemble into monodisperse nanocrystals. A newer, simpler strategy uses preformed nanocrystals as templates and chemical transformation to change the composition. Solution-based mechanisms can chemically transform nanomaterials, allowing atoms to be easily and precisely incorporated, removed, or replaced from preformed templates. The approach uses oxidation, reduction, alloying, or atomic exchange reactions. In ionic nanocrystals, cation exchange can be driven by solvation energy differences between template and solvated ions. Ion solubilities can be controlled by adding selective coordinating species to the solution. In metal nanocrystals, atomic exchange reactions reflect reduction potential differences between the template metal and solvated metal ions. This galvanic replacement method involves a Wikipedia:redox reaction. Placing a nanocrystal in a solution containing metal ions with a higher reduction potential oxidizes the templates' surface, dissolving its metal ions. The released electrons reduce the ions from the solution, which deposit at the template's surface. Galvanic replacement also applies to ionic compounds. In oxide nanocrystals, a redox-couple reaction can occur between multivalent metallic ions. E.g., higher–oxidation state ions in Wikipedia:manganese oxide nanocrystals have been replaced with solvated lower–oxidation state iron ions. Atomic diffusion is a key parameter in such reactions. Chemical transformation tools provide complete composition control only within the Wikipedia:atomic diffusion length. High nanocrystal surface-to-volume ratios expose the entire lattice to diffusion. The effective particle size range for these tools depends on the material, but can reach hundreds of nanometers. Applications Nanocrystals made with Wikipedia:zeolite are used to filter crude oil onto diesel fuel at an Wikipedia:ExxonMobil Wikipedia:oil refinery in Wikipedia:Louisiana at a cost less than conventional methods. Nanocrystalline material A NC material is a polycrystalline material with a Wikipedia:crystallite size of only a few Wikipedia:nanometers. These materials fill the gap between Wikipedia:amorphous materials without any Wikipedia:long range order and conventional course-grained materials. Definitions vary, but nanocrystalline material is commonly defined as a Wikipedia:crystallite (grain) size below 100 nm. Grain sizes from 100–500 nm are typically considered "ultrafine" grains. The grain size of a NC sample can be estimated using Wikipedia:x-ray diffraction. In materials with very small grain sizes, the diffraction peaks will be broadened. This broadening can be related to a crystallite size using the Wikipedia:Scherrer equation (applicable up to ~50 nm), a Williamson-Hall plot, or more sophisticated methods such as the Warren-Averbach method or computer modeling of the diffraction pattern. The crystallite size can be measured directly using Wikipedia:transmission electron microscopy. Synthesis Nanocrystalline materials can be prepared in several ways. Methods are typically categorized based on the phase of matter the material transitions through before forming the nanocrystalline final product. Solid-state processing Solid-state processes do not involve melting or evaporating the material and are typically done at relatively low temperatures. Examples of solid state processes include Wikipedia:mechanical alloying using a high-energy ball mill and certain types of Wikipedia:severe plastic deformation processes. Liquid processing Nanocrystalline metals can be produced by rapid Wikipedia:solidification from the liquid using a process such as Wikipedia:melt spinning. This often produces an amorphous metal, which can be transformed into an nc metal by annealing above the Wikipedia:crystallization temperature. Vapor-phase processing Wikipedia:Thin films of nanocrystalline materials can be produced using Wikipedia:vapor deposition processes such as Wikipedia:MOCVD. Solution processing Some metals, particularly Wikipedia:nickel and Wikipedia:nickel alloys, can be made into nanocrystalline foils using electrodeposition. thumb|right|350px|Nanocrystal channel design. Nanocrystal solar cells Wikipedia:Solar cells based on a substrate with a Wikipedia:coating of Wikipedia:nanocrystals. The nanocrystals are typically based on Wikipedia:silicon, Wikipedia:CdTe or CIGS and the substrates are generally silicon or various organic conductors. Wikipedia:Quantum dot solar cells are a variant of this approach, but take advantage of quantum mechanical effects to extract further performance. Wikipedia:Dye-sensitized solar cells are another related approach, but in this case the nano-structuring is part of the substrate. Previous fabrication methods relied on expensive Wikipedia:molecular beam epitaxy processes, but colloidal synthesis allows for cheaper manufacture. A thin film of nanocrystals is obtained by a process known as “spin-coating”. This involves placing an amount of the quantum dot solution onto a flat substrate, which is then rotated very quickly. The solution spreads out uniformly, and the substrate is spun until the required thickness is achieved. Quantum dot based photovoltaic cells based around dye-sensitized colloidal TiO2 films were investigated in 1991 and were found to exhibit promising efficiency of converting incident light energy to electrical energy, and to be incredibly encouraging due to the low cost of materials used. A single-nanocrystal (channel) architecture in which an array of single particles between the electrodes, each separated by ~1 exciton diffusion length, was proposed to improve the device efficiency (figure below) and research on this type of solar cell is being conducted by groups at Stanford, Berkeley and the University of Tokyo. Although research is still in its infancy, in the future nanocrystal photovoltaics may offer advantages such as flexibility (quantum dot-polymer composite photovoltaics ) lower costs, clean power generation and an efficiency of 65%,Quantum Dots May Boost Photovoltaic Efficiency To 65% compared to around 27% for first-generation photovoltaics. thumb|Efficiency of different solar cells. It is argued that many measurements of the efficiency of the nanocrystal solar cell are incorrect and that nanocrystal solar cells are not suitable for large scale manufacturing. Recent research has experimented with Wikipedia:lead selenide (PbSe) semiconductor, as well as with Wikipedia:cadmium telluride (CdTe), which has already been well established in the production of "classic" solar cells. Other materials are being researched as well. Other third generation solar cells *Wikipedia:Photoelectrochemical cell *Wikipedia:Polymer solar cell Wikipedia:Template:Photovoltaics Light-Emitting spectrum-adjustable nanocrystals Turkish Wikipedia:scientists conducted research into doped nanocrystals; they were led by Wikipedia:Hilmi Volkan Demir at Wikipedia:Bilkent University, located in the Turkish capital Wikipedia:Ankara. They succeeded in creating a new type of nanocrystals; white light is generated by controlled layer-by-layer assembly of nanocrystals on Wikipedia:nitride diodes. All spectra of light can be generated, making white light possible, but the parameters of the light color can also be adjusted. CdSe/ZnS core–shell nanocrystals of different sizes are hybridized with InGaN/GaN Wikipedia:LEDs for this purpose.http://iopscience.iop.org/0957-4484/18/40/405702 1; 2; 3; 4; 5; 6 See also * Cadmium telluride nanocrystals * Wikipedia:Magnetic nanoparticles * Wikipedia:Nanocrystal solar cell * Wikipedia:Quantum dot * Wikipedia:Quantum dot solar cell * Wikipedia:Nanocrystal * Wikipedia:Nanocrystalline silicon * Wikipedia:Nanoparticle Wikipedia:Portal:Renewable energyWikipedia:Portal:Energy External links * *Science News Online, Quantum-Dots Leap: Tapping tiny crystals' inexplicable light-harvesting talent, June 3, 2006. *Wikipedia:InformationWeek, Nanocrystal Discovery Has Solar Cell Potential, January 6, 2006. *Wikipedia:Berkeley Lab, Berkeley Lab Air-stable Inorganic Nanocrystal Solar Cells Processed from Solution, 2005. *ScienceDaily, Sunny Future For Nanocrystal Solar Cells, October 23, 2005. * Elan Drug Technology * Nanocrystalline Soft Magnetic Materials * Jose-Yacaman article on gold/silver nanocrystals Tunabl W-L-E * Personal web site of Hilmi Volkan Demir References